Flying any aircraft requires knowledge of its airspeed, that is to say of its speed in relation to the relative wind. This speed is determined using sensors that sense the static pressure Ps, the total pressure Pt, the angle of attack α and the side-slip angle β. α and β provide the direction of the velocity vector in a reference system, or frame of reference, associated with the aircraft, and Pt-Ps provides the modulus of this velocity vector. The four aerodynamic parameters therefore make it possible to determine the velocity vector of an aeroplane and, as a side issue, of a tilt-rotor aircraft of the so-called “convertible” type.
The total pressure Pt is usually measured using a tube known as a Pitot tube. This is a tube that is open at one of its ends and closed at the other. The open end of the tube faces substantially into the flow.
Inside the Pitot tube, near the closed end, is located a means for measuring the air pressure therein. The stream of air situated upstream of the tube is gradually slowed until it reaches a zero speed at the entrance to the tube.
The slowing of the speed of the air tends to increase the air pressure. This increased pressure forms the total pressure Pt of the airflow.
In practice, the airflow may carry solid or liquid particles liable to enter the Pitot tube and to build up in the tube at the closed end. In order to avoid such a build-up disrupting the pressure measurement, the closed end generally has a drain-hole through which any particles can be removed.
Both the particles and some of the air that has entered the Pitot tube flow through this hole. Hence, the slowing of the air in the tube is incomplete and the total pressure Pt measurement is corrupted thereby. More specifically, the more effort is made in avoiding the build-up of large-sized particles, the more the total pressure measurement is corrupted as the dimensions of the drain-hole increase.
Conversely, the more effort is put into improving the total pressure Pt measurement by decreasing the dimensions of the drain-hole, the more the risk of a build-up of particles increases.
With a Pitot tube, it is therefore necessary to reach a compromise between the quality of the total pressure Pt measurement and the risk of the measurement being disrupted as a result of particles carried by the airflow where the measurement is taken.
In order to lessen the effects of this shortcoming a probe that measures total pressure at a stagnation point is used. More specifically, this probe taps off at least two air streams from an airflow and brings them into contact with one another in order to slow them down. The pressure in the zone where the air is slowed is measured and this measured pressure gives the total pressure of the flow. A probe such as this is described in French patent application No. FR 2 823 846.
The two types of probe, Pitot tube and stagnation point probe, are not susceptible to the same risks of blockage. Pitot tubes are susceptible to small-sized particles that might block their drain-hole. Stagnation point probes are susceptible to larger-sized particles, such as a bird, for example, obstructing the zone at which the air streams are tapped off.